Functional and effective T-cell responses play an important role in effective immune responses, for example, against infectious diseases and cancer. However, under certain conditions, such as chronic infection or cancer, effector T cells can be suppressed by various immunosuppressive mechanisms, including programmed death ligand-1 (PD-L1)/programmed death-1 (PD-1) interaction, leading to T-cell exhaustion (Pen et al. Gene Therapy 21, 262-271, 2014). It is thought that PD-L1 is normally expressed by most cell types, while its receptor PD-1 is only present on certain immune cells, such as activated T cells and regulatory T (Treg) cells. It is also thought that PD-L1/PD-1 binding is important in the maintenance of peripheral T-cell tolerance, preventing auto immune responses. On the other hand, high levels of PD-1 expression generally correlate with loss of T cell function, leading to increased viral load in cases of viral infection (Pen et al. Gene Therapy 21, 262-271, 2014).